class Solution
{
public:
    bool hasCycle(ListNode* head)
    {
        if (head == nullptr || head->next == nullptr)
        {
            return false;
        }
        ListNode* slow = head;
        ListNode* fast = head->next;
        while (slow != fast)
        {
            if (fast == nullptr || fast->next == nullptr)
            {
                return false;
            }
            slow = slow->next;
            fast = fast->next->next;
        }
        return true;
    }
};
